WhenWithOnlyElse: don't remove when subject variable if needed

#KT-30975 Fixed

idea/testData/inspectionsLocal/whenWithOnlyElse/subjectVariable/blockElse/singleReference.kt

@@ -0,0 +1,14 @@
+fun test() {
+    when (val a = create()) {
+        else -> {
+            use(a)
+            foo()
+        }
+    }<caret>
+}
+
+fun create(): String = ""
+
+fun use(s: String) {}
+
+fun foo() {}